Update tasklist

[ashygee]

This PR updates the following:

• modifies tasklist icon proposed in https://github.com/github/primer/issues/746
• adds a new checkbox icon using metaphor from previous tasklist metaphor

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 28257649bc2aeb86391dbd9468d91be01b285289

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ---------------- | ----- | | @primer/octicons | Minor |

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